Copper Complexes in Radical Photoinitiating Systems: Applications to Free Radical and Cationic Polymerization upon Visible LEDs

Three copper complexes (E1, G1, and G2) with different ligands in combination with an iodonium salt (and optionally another additive) were used to generate radicals upon soft visible light exposure (e.g., polychromatic visible light from a halogen lamp, laser diodes at 405 and 457 nm, LEDs at 405 and 455 nm). This approach can be worthwhile and versatile to initiate free radical photopolymerization, ring-opening cationic photopolymerization, and the synthesis of interpenetrating polymer networks. The photochemical mechanisms for the production of initiating radicals are studied using cyclic voltammetry, electron spin resonance spin trapping, steady state photolysis, and laser flash photolysis techniques. The photoinitiation ability of the copper complexes based photoinitiating systems are evaluated using real-time Fourier transform infrared spectroscopy. G1 and G2 are better than the well-known camphorquinone (CQ)-based systems (i.e., TMPTA conversion = 18%, 35%, 48%, and 39% with CQ/iodonium salt, CQ/amine, G1/iodonium salt, and G2/iodonium salt, respectively; halogen lamp exposure). Interestingly, some of these systems are also better than the well-known type I phosphine oxide photoinitiator (BAPO) clearly showing their high performance. These copper complexes can be used as highly efficient catalysts in photoredox catalysis.